Large, high pressure fluid swivels are commonly used in offshore hydrocarbon floating production systems. Such a fluid swivel commonly has a sealing surface diameter of about one to ten meters and carries fluid at a working pressure on the order of magnitude of a few hundred psi (a few hundred mega pascals). In such a system, fluids such as oil, natural gas, and water are transferred between an undersea well and a ship, the fluid swivel allowing the ship to weathervane. The fluid swivel includes inner and outer ring-shaped swivel parts rotating on one another, with a transfer chamber that is usually annular formed between them and with a pair of gap passages extending from the chamber to the environment. One or more seal devices lie along each gap passage, in a ring-shaped cavity, to prevent leakage of pressurized fluid.
The pressure of fluid passing through the fluid swivel may vary greatly. It would be desirable if the force with which the seal device pressed against the sealed surface that moves relative to the seal device, could be adjusted. This would result in the seal device wearing at a high rate only when high pressured fluid had to be sealed, to extend the life of the seal device and minimize friction under low pressure conditions while enabling sealing at high pressure.